Corpus Luteum (CL)

The corpus luteum (CL) is a short-lived, hormone-producing gland that forms from the follicle after ovulation. It produces the progesterone that the endometrium, the lining of the uterus, needs to prepare for implantation. It also produces a moderate amount of estradiol, a form of estrogen. When the corpus luteum does not function well, the lining cannot support implantation. An early pregnancy then cannot be sustained.¹

When an egg is fertilized, the outer cells of the implanting embryo (trophoblast) release human chorionic gonadotropin (hCG). The hCG binds to the CL and rescues it from its programmed breakdown.² The corpus luteum continues to produce progesterone until the placenta takes over. This transition is usually complete by about 7 to 9 weeks of pregnancy.

When no egg is fertilized, the CL breaks down over 10 to 14 days. Progesterone falls, and menstruation follows. The length and hormone output of the luteal phase directly reflect how well the corpus luteum functions.

Poor CL function is the cause of Corpus Luteum Deficiency (CLD). CLD is a distinct type of luteal phase deficiency. RRM clinicians may assess CL function with progesterone blood tests timed to the days after Peak.

A chart can look normal and still show a short or poorly supported post-Peak phase. This pattern is a clinical signal. The corpus luteum reflects the events that preceded it. These include how the follicle developed, the hormonal environment of the cycle, and the strength of the LH surge. Treatment of CL deficiency that ignores these earlier factors addresses the signal but not the source.